Causes
Algae growth causes a dark discoloration on roofs. The algae is usually brown to black in color which results in a streaked, dirty looking rooftop. It can be mistaken for soot, dirt or tree droppings, which typically produce only localized discoloration. It's caused by the growth of air-borne algae which deposits on your roof. The good news is that algae growth does not affect the service life of your roofing material. It occurs most often in warm, humid climates such as the Southeast (where algae-resistant shingles are available).

Recommendations
Discoloration of roofs caused by fungus or algae is difficult to remove, but may be lightened by spraying a diluted solution of chlorine bleach, trisodium phosphate, and water onto the roof. Solutions range from one cup TSP, one gallon bleach and four gallons of water, to one cup TSP and 2.5 gallons each of bleach and water. This solution can then be rinsed from the roof in 10 to 15 minutes. Do not scrub. Scrubbing will loosen and remove granules. After spraying be sure to thoroughly flush the roof and the greenery around the home (grass, bushes and shrubs) with water. The effectiveness of this type of cleaning may only be temporary, and the discoloration may recur. High pressure washing is not recommended due to the possibility of removing granules, thereby shortening the life of the Roofing System.